All Questions
76
questions
2
votes
1
answer
281
views
Negative Horizon distance
Consider a flat universe, here, proper distance can be given by R-W Metric:
$$d_p (t_0) = c\int_{t_e}^{t_0}\frac{dt}{a(t)},$$ $t_e$ is the time when a photon is emitted from a distant galaxy, $t_0$ is ...
- 411
0
votes
1
answer
64
views
Horizon problem, what if our observable universe is roughly equal to the whole universe, especially in early times?
How do we know that at a time of 380.000 years, when CMB got free, the observable universe was not equal to the actual universe? Maybe they were roughly the same and couldn't that explain the horizon ...
- 1
4
votes
3
answers
2k
views
If the observable universe had only one galaxy, how would people know the expansion of the universe?
Hubble measured high redshifted galaxies to discover the cosmic expansion. In a hypothetical universe where only one galaxy exists, would there still be observational evidence for the Big Bang theory? ...
- 195
0
votes
0
answers
33
views
What was the size of the observable universe at the matter-dominated era and its mass density?
For instance, at the scale factor $a=0.5$ and matter dominated era, what was the size of the observable universe and matter density?
- 476
0
votes
1
answer
77
views
How a star can move out from visible universe? [duplicate]
We see only stars whose light is not older than the age of universe. This is understood. If a star is too far away that currently we don't see it then possibly we will see it later, then the age of ...
- 362
0
votes
1
answer
74
views
Hubble expansion across time dimension?
The expansion of the universe, as described by the Hubble's law, refers to the expansion of space itself. This expansion is observed in the three spatial dimensions (length, width, and height), rather ...
- 101
0
votes
0
answers
55
views
Big Bang and where do we stand relative to it? [duplicate]
Maybe dumb question, not sure, but I would need some help here to understand.
https://theglobestalk.com/james-webb-telescope-see-back-in-time/
So according to physics we can look back in time ...
1
vote
0
answers
33
views
Can we infer the size of the whole universe from its expansion rate? [closed]
If the universe inflated to 100 billion km in its first second, that suggests only 1/160,000 of it was observable from any point at that moment. The expansion rate slowed after that, of course, but ...
- 384
0
votes
1
answer
55
views
Light emitted at $t = 0$
So solving for the scale factor from the Friedmann equation we can then use it to calculate proper distance via
$$d_p(t_o) = c \int_{t_e}^{t_o} \frac{dt}{a(t)}$$
For a particular universe $a(t)$ is ...
- 161
0
votes
1
answer
138
views
When will the particle horizon reach its limit of 63 billion light years?
In the far future, the particle horizon will reach 63 Gly，so when will that time? Just like today's universe time is 13.82 Gyr and the particle horizon is 46.5 Gly, how many years will the particle ...
-4
votes
1
answer
120
views
Cosmos at minimum 250x bigger than our observable Universe, so why then the need for a Big Bang?
Please correct me If I'm wrong but does not the BB only refers to our light speed limited observable Universe (OU) from our home position?
Also it is estimated that the Cosmos is minimum 250 times the ...
- 4,170
4
votes
1
answer
663
views
Why don't we see the big bang?
excuse my understanding, my brain is melting.
So I understand that pictures of far away objects is like viewing the past.
and I think I pretty much get that the big bang was in every direction since ...
- 973
1
vote
2
answers
153
views
Can the age of the universe be much bigger than 13.8 billion
If observable universe is only a small fraction of the existing universe, does it imply that the age of the universe is much more than 13.8 billion years or the expansion of the universe is much ...
- 57
1
vote
0
answers
54
views
How is the expansion of the universe measured if redshift depends on the expansion itself? [duplicate]
To me this seems like a bit of a chicken egg problem.
Based on the redshift of light (plus the assumption that physics worked the same way back then and there as it does now and here), we can ...
2
votes
1
answer
155
views
Why do some diagrams of the particle horizon, observable universe etc show the past light cone as hitting ~20glyr out from us at time zero?
This question is mainly in reference to this question:
Is the observable universe equivalent to 'our' light cone?,
and the answer, which is great.
But what I can't wrap my head around is why ...
- 21